[SLP] Fix spill-cost cache lookup and predecessor scan
A cached intra-block scan that stopped at a call or budget limit only
proves the sub-range below the stop point is call-free; do not reuse
the cached bit for queries whose First lies above it. Also switch the
cross-block predecessor scan to "exists a call-free backward path"
semantics, skip blocks strictly dominated by Root, and memoize only
the (Root, OpParent) key. Fixes a false-positive spill cost that was
blocking profitable vectorization.
Reviewers: RKSimon, hiraditya, bababuck
Pull Request: https://github.com/llvm/llvm-project/pull/192709
